Efficient oceanic operations normally require flight level changes. Climbs or descents provide optimal performance to take advantage of favorable winds or to avoid turbulence.
Current oceanic operations limit opportunities for flight level changes for a number of reasons:                Flights operate along same routes at about the same time (locally dense traffic);        Reduced surveillance performance (compared with radar) results in large separation minima for safe procedural separation.        
Automatic dependent surveillance-broadcast (ADS-B) in-trail procedures (ITP) are airborne ADS-B enabled climbs and descents through otherwise blocked flight levels. ITP is based on an approved International Civil Aviation Organization (ICAO) procedure whereby a controller separates aircraft based on information derived from cockpit sources that is relayed by the flight crew.
ITP allows a leading or following aircraft on the same track to climb or descend to a desired flight level through flight levels occupied by other aircraft. An ITP display enables a flight crew to determine if specific criteria for an ITP are met with respect to one or two reference aircraft at intervening flight levels. These criteria ensure that the spacing between the estimated positions of the ITP aircraft and reference aircraft always exceeds the ITP separation minimum of 10 NM, while vertical separation does not exist during the climb or descent. Once the flight crew has established that the ITP criteria are met, they request an ITP climb or descent, identifying any reference aircraft in the clearance request. Air Traffic Control (ATC) must determine if standard separation will be met for all aircraft at the requested flight level—and at all flight levels between the initial flight level and requested flight level. If so, a standard (non-ITP) flight level change clearance is likely to be granted. Otherwise, if the reference aircraft are the only blocking aircraft, the controller evaluates the ITP request. ATC determines if the reference aircraft have been cleared to change speed or change flight level, or are about to reach a point at which a significant change of track will occur. The controller also ensures that the requesting aircraft is not referenced in another procedure. ATC also ensures that the positive Mach difference with the reference aircraft is no greater than 0.06 Mach. If each of these criteria are satisfied, then ATC may issue the ITP flight level change clearance.
An example of an ITP climb is shown in FIGS. 1 and 2. An ITP aircraft is behind a reference aircraft that is at a higher intervening flight level (FL350). Standard air traffic control (ATC) procedures apply to the other aircraft (two aircraft at FL360 and one at FL350).
ITP requires new airborne equipment to provide improved information about nearby traffic. ADS-B data broadcast from these aircraft provide more accurate position data than currently available to oceanic controllers. The more accurate airborne surveillance data facilitate safe flight level changes through intervening flight levels. The airborne ITP system receives ADS-B data that includes flight identification, altitude, aircraft position, groundspeed and quality-of-data information. The ITP system displays the information derived from received ADS-B data on traffic displays such as a cockpit display of traffic information (CDTI). Both plan-view and vertical situational awareness displays (VSAD) are possible, see FIG. 3.